SALT LAKE CITY, Feb. 11, 2010 – Humans, other great apes
and bears are among the few animals that step first on the heel
when walking, and then roll onto the ball of the foot and toes.
Now, a University of Utah study shows the advantage: Compared with
heel-first walking, it takes 53 percent more energy to walk on the
balls of your feet, and 83 percent more energy to walk on your
toes.
"Our heel touches the ground at the start of each step. In most
mammals, the heel remains elevated during walking and running,"
says biology Professor David Carrier, senior author of the new
study being published online Friday, Feb. 12 and in the March 1
print issue of The Journal of Experimental Biology.
"Most mammals – dogs, cats, raccoons – walk and run
around on the balls of their feet. Ungulates like horses and deer
run and walk on their tiptoes," he adds. "Few species land on their
heel: bears and humans and other great apes – chimps,
gorillas, orangutans."
"Our study shows that the heel-down posture increases the
economy of walking but not the economy of running," says Carrier.
"You consume more energy when you walk on the balls of your feet or
your toes than when you walk heels first."
Economical walking would have helped early human
hunter-gatherers find food, he says. Yet, because other great apes
also are heel-first walkers, it means the trait evolved before our
common ancestors descended from the trees, he adds.
"We [human ancestors] had this foot posture when we were up in
the trees," Carrier says. "Heel-first walking was there in the
great apes, but great apes don't walk long distances. So economy of
walking probably doesn't explain this foot posture [and why it
evolved], even though it helps us to walk economically."
Carrier speculates that a heel-first foot posture "may be
advantageous during fighting by increasing stability and applying
more torque to the ground to twist, push and shove. And it
increases agility in rapid turning maneuvers during aggressive
encounters."
The study concludes: "Relative to other mammals, humans are
economical walkers but not economical runners. Given the great
distances hunter-gatherers travel, it is not surprising that humans
retained a foot posture, inherited from our more arboreal
[tree-dwelling] great ape ancestors, that facilitates economical
walking."
Measuring the Costs of Different Modes of Walking and
Running
Carrier conducted the study with Christopher Cunningham, a
doctoral student in biology at the University of Utah; Nadja
Schilling, a zoologist at Friedrich Schiller University of Jena,
Germany; and Christoph Anders, a physician at University Hospital
Jena. The study was funded by the National Science Foundation,
Friedrich Schiller University of Jena and a German food industry
insurance group interested in back pain.
The study involved 27 volunteers, mostly athletes in their 20s,
30s and 40s. Each subject walked or ran three different ways, with
each step either heel-first, ball-of-foot first with the heel a bit
elevated or toes first with the heel even more elevated.
In his lab, Carrier and colleagues measured oxygen consumption
– and thus energy use – as 11 volunteers wore face
masks while walking or running on a treadmill. They also walked on
a "force plate" to measure forces exerted on the ground.
Part of the study was conducted at Anders' lab in Germany, where
16 people walked or ran on a treadmill as scientists monitored
activity of muscles that help the ankles, knees, hips and back do
work during walking and running.
Findings of the experiments included:
- "You consume more energy when you walk on the balls of your
feet or your toes than when you walk heels-first," Carrier says.
Compared with heels-first walkers, those stepping first on the
balls of their feet used 53 percent more energy, and those stepping
toes-first expended 83 percent more energy.
- "The activity of the major muscles of the ankle, knee, hip and
back all increase if you walk on the balls of your feet or your
toes as opposed to landing on your heels," says Carrier. "That
tells us the muscles increase the amount of work they are producing
if you walk on the balls of your feet."
- "When we walk on the balls of our feet, we take shorter, more
frequent strides," Carrier says. "But this did not make walking
less economical." Putting the heel down first and pivoting onto the
ball of the foot makes the stride longer because the full length of
the foot is added to the length of the step. But that has no effect
on energy use.
- The researchers wondered if stepping first on the balls of the
feet took more energy than walking heel-first because people are
less stable on their toes or balls of the feet. But increased
stability did not explain why heel-first walking uses less
energy.
- Stepping heel-first reduced the up-and-down motion of the
body's center of mass during walking and required less work by the
hips, knees and ankles. Stepping first onto the balls of the feet
slows the body more and requires more re-acceleration.
- Heels-first steps also made walking more economical by
increasing the transfer of movement or "kinetic" energy to stored
or "potential" energy and back again. As a person starts to step
forward and downward, stored energy is changed to motion or kinetic
energy. Then, as weight shifts onto the foot and the person moved
forward and upward, their speed slows down, so the kinetic energy
of motion is converted back into stored or potential energy. The
study found that stepping first onto the balls of the feet made
this energy exchange less efficient that walking heels-first.
- Heel-first walking also reduced the "ground reaction force
moment" at the ankle. That means stepping first onto the ball of
the foot "decreases the leverage, decreases the mechanical
advantage" compared with walking heel-first, Carrier says.
In sum, walking heel-first is not more economical because it is
more stable or involves fewer, longer strides, but because when we
land on our heels, less energy is lost to the ground, we have more
leverage, and kinetic and potential energy are converted more
efficiently.
Form and Function of the Foot
If heel-first walking is so economical, why do so many animals
walk other ways?
"They are adapted for running," Carrier says. "They've
compromised their economy of walking for the economy of
running."
"Humans are very good at running long distances. We are
physiologically and anatomically specialized for running long
distances. But the anatomy of our feet is not consistent with
economical running. Think of all the animals that are the best
runners – gazelles, deer, horses, dogs – they all run
on the ball of their feet or the tips of their toes."
When people run, why is there no difference in the amount of
energy they expend when stepping first onto their heels versus the
balls of their feet or toes?
The answer is unknown, but "if you land on your heel when you
run, the force underneath the foot shoots very quickly to the ball
of your foot," Carrier says. "Even when we run with a heel plant,
most of the step our weight is supported by the ball of our foot.
Lots of elite athletes, whether sprinters or distance runners,
don't land on their heel. Many of them run on the balls of their
feet," as do people who run barefoot. That appears to be the
natural ancestral condition for early human runners, he adds.
"The important thing is we are remarkable economical walkers,"
Carrier says. "We are not efficient runners. In fact, we consume
more energy to run than the typical mammal our size. But we are
exceptionally economical walkers."
"This study suggests that one of the things that may explain
such economy is the unusual structure of our foot," he adds. "The
whole foot contacts the ground when we walk. We have a big heel.
Our big toe is as long as our other toes and is much more robust.
Our big toe also is parallel to and right next to the second
toe."
"These features are distinct among apes, and provide the
mechanical basis for economical walking. No other primate or mammal
could fit into human shoes."
SOURCE